<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .line{
            height: 2px;
            width: 600px;
            background-image: linear-gradient(
                to right,
                transparent,
                #666,
                transparent
            );
        }
    </style>
</head>
<body>
    <div class="line"></div>
</body>
</html>